Field of the Invention
The present invention relates to a configuration for determining the concentration of contaminating particles in a loading and unloading chamber of an appliance for processing at least one disk-like object.
In semiconductor fabrication, contamination, for example by particles or foreign substances, constitutes a great risk with the consequences of reducing the quality and total failure of the electronic components. For this reason, the environmental conditions during semiconductor fabrication are kept at the highest possible quality level by filters and by monitoring the physical conditions in the room. That is, the number of contaminating particles per volume element is kept as low as possible.
In order to meet the requirements on structure widths which decrease to an ever increasing extent, a recent development includes much better clean conditions, as compared with the clean room condition (already improved in any case), which are created by using mini environments locally in the processing appliances or in the containers for the semiconductor products (for example, for semiconductor wafers, masks or flat panel displays). As compared with the surrounding clean room, the air in the mini environments has a much lower number of contaminating particles per volume. In particular, in this case, the loading and unloading areas of appliances for processing the disk-like objects are configured as mini environments. This is correspondingly true of appliances for transferring or resorting the products within the transport containers, for example “stockers” or “rake transfer” appliances.
Unfortunately, all moveable parts and components produce contamination, even during fault-free operation. The handling systems for disks and masks in semiconductor fabrication are primarily considered. Unacceptable contamination can occur if maladjustments of the handling systems arise, for example, when disks being loaded or unloaded in a “disk carrier” and “boats” are scraped as a result of an inaccurate adjustment; in the process, layers on the disk or on the disk carrier flake off and become a contamination source. The particles flaked off can be deposited on the same disk or on the disks, which follow or are located underneath and lead to yield losses in the latter.
In particular, disk carriers or boats themselves constitute a serious contamination source. Typically, such disk carriers are used during the deposition of layers on semiconductor disks and wafers. During batch processing, a large number of semiconductor wafers are introduced simultaneously into the insertion spaces (slots), which are normally stacked vertically one above another and, together with the disk carriers, are processed in a processing chamber. Therefore, not only are the semiconductor wafers, but also the disk carriers are coated with the material respectively deposited.
Since the disk carriers are neither cleaned immediately nor replaced following the respective process, the layer thickness accumulates in the course of several processes. As a result of thermal stress or vibration during loading or unloading of the disks, layer particles begin to flake off to an increasing extent after a specific layer thickness has been reached. Thermal stresses are produced in particular, since the disk carriers cool down following oven processing for “layer deposition”. The (moving-out) disk carriers still have temperatures of several hundred degrees in the comparatively cool loading and unloading area of the fabrication appliances. A further cause is constituted by sudden pressure changes following low-pressure or high-pressure processes.
Therefore, the loading and unloading areas of processing appliances in semiconductor fabrication are subject to particularly high contamination risk, as a result of particles flaking off, both as a result of increased object handling and as a result of the severe changes in the physical environmental conditions.
The problem is normally monitored by the simultaneous processing of test disks. Following processing by the appliances, the test disks are subjected to a surface inspection to determine particle numbers. If, violations of specified limiting values are determined, a search for the cause can be made and, either maladjustment of the handling systems can be corrected or, if disk carriers are used, they can be replaced.
Further, the following problems may arise: firstly, the processing and examination of test disks leads to a loss of fabrication capacity. Secondly, as a result of the late measurement result, contamination problems that occur suddenly are not detected immediately. In particular, in the case of the problem of the growing “layer thickness” on the disk carriers, the property of the layer material (i.e., “flake off” property) can occur relatively abruptly. In addition, maladjustment of the handling systems can occur suddenly, for example, as a result of external vibrations.
Therefore, in the case of the disk carriers, the accumulated layer thickness is logged and, in order to avoid contamination problems, replacements are made precisely at the point when empirically determined layer thicknesses are reached, at which the flaking of the layer material will supposedly occur. As a result of early flaking (i.e., before the specific layer thickness is reached), a contamination problem arises, which threatens the yield and may be undiscovered. Still, the layer on the disk carriers can be stable, even beyond the determined layer thickness. Nevertheless, since the disk carrier is replaced, a potential “saving” remains unused.